Communication lines, suitable for allowing the transmission of communications over great distances, usually provide, in addition to the channels used for communication signals provided for subscribers, an independent channel, suitable for allowing the transmission of service communications.
Such service signals can be of different kinds, e.g., for control or command signals for equipment located along the line, such as amplifiers or repeaters, or for communications between maintenance staff, located at a point along the line, and an intermediate or end station of the line itself.
In an optical fiber telecommunication line, provided at regular intervals with repeaters for the amplification of the transmitted signals, one or more of the channels may be used for the service signals, which are accessible, for sending or receiving service signals, at each repeater, where the optical signals are detected and transformed into electrical signals, electronically amplified and once again sent towards the destination station, in an optical form.
In such repeaters, a service signal converted into an electrical form can be easily received and used for the desired purposes, and in a similar manner, a signal may be injected in an electric form into the repeater and then converted into an optical signal together with the other signals subjected to amplification and sent along the line.
However, optical fiber telecommunication lines which have currently proved convenient and which, instead of repeaters, of an electronic type, use optical amplifiers, in a position of amplifying the signal without converting it into an electrical form.
In such lines, it is not possible for signals to be injected into or to be extracted from the fiber along which they are transmitted with the known electronic equipment, because the signals are available only in an optical form, even at the amplifiers.
The problem, therefore, arises of injecting into and extracting the service signals from an optical telecommunications line, possibly provided with optical amplifiers, by operating on the signals themselves in an optical form.
There are known devices called "optical couplers" which are suitable for injecting signals into, or for extracting signals from, an optical fiber, such signals having a wavelength different from that of the other signals which pass unaltered, but, in order for such couplers to operate correctly, with a complete separation between the extracted signals and the unaltered signals and with a reduced attenuation of the signals themselves, they must operate between wavelengths that are substantially different whereas optical communications are accomplished in a fairly narrow range of wavelengths where the transmission characteristics of the fiber are better.